(1) Field of the Invention
The present invention relates generally to quiet undersea vehicles and more particularly to undersea vehicles having internal or ducted propulsor systems.
(2) Description of the Prior Art
It is well known that control surface actuator noise, as well as turbulence induced noise created by the interaction of propellers and control surfaces, are significant sources of unwanted noise on undersea vehicles, such as torpedoes and unmanned undersea vehicles. Present control surfaces and propulsor configurations have unacceptably high acoustic noise levels. A variety of techniques have been used to reduce the amount of noise created by existing electromechanical actuators. In general, these efforts have concentrated on balancing and isolating the moving parts and gears as well as providing fixed hydrodynamic fairings to minimize turbulence-induced noise. Unfortunately, even in the best prior art designs, electromechanical actuator-driven control surfaces suffer from several drawbacks. Actuation of the control surfaces result in gear and motor noise. Further, these control surfaces, typically located ahead of the propellers, create a turbulent wake behind the control surfaces. The ingestion of this wake by the propellers generates significant flow noise levels. The flow noise is created by three mechanisms: (1) the turbulence directly radiating to the near and far field, (2) the induced noise due to the turbulent excitation of the control surface and the surrounding structure, and (3) interaction of the control surface wake with the propulsor. The third item causes fin and structure re-radiation which is the dominant flow noise source.
Additionally, in remotely-operated undersea vehicles, used in surveillance or reconnaissance, a low reflectivity profile is needed to avoid active sonar detection. The conventional structure of fins, control surfaces and propellers creates multiple corner reflectors resulting in very strong return echoes. An undersea vehicle used in covert surveillance must have a minimum of external structure for controls and propulsion.